Bisguanidinium-Catalyzed Epoxidation of Allylic and Homoallylic Amines under Phase Transfer Conditions

A highly enantioselective
epoxidation reaction of allylic and homoallylic
amines has been disclosed using an ion pair catalyst, which consists
of chiral cationic bisguanidinium [BG]2+ and
an achiral tetraperoxyditungstate anion [W2O2(μ-O)­(O2)4]2–. The
terminal oxidant is a stoichiometric amount of aqueous hydrogen peroxide,
an environmentally benign reagent. Up to 96% enantiomeric excess and
99% yields were achieved for 1,1′-disubstituted and 1,2-disubstituted
allylic protected amines and 1,2-disubstituted homoallylic protected
amines. The identity of the ion pair catalyst was uncovered using
X-ray crystallography and revealed that the achiral tetraperoxyditungstate
anion species [W2O2(μ-O)­(O2)4]2– is nudged nicely into the central
cavity of the chiral dication. The ion pair catalyst was also characterized
using infrared (IR) and Raman spectroscopies. The synthesis of (−)-venlafaxine
was achieved via this reported methodology to demonstrate its usefulness.